There's never been a better time to be a high-end system builder. Intel's Skylake Server core made its way onto uber-desktops back in June as the Skylake-X family of chips, and AMD returned the serve with its Ryzen Threadripper CPUs and the X399 platform. Now it's Intel's turn to raise the stakes again.
On the bench today, we have the 16-core, 32-thread Core i9-7960X and the newest Extreme Edition CPU: the 18-core, 36-thread Core i9-7980XE. The Core i9-7960X is a core-for-core, thread-for-thread match against the Ryzen Threadripper 1950X, while the Core i9-7980XE lays claim to what is perhaps the highest core and thread count available in a "consumer" CPU today.
Of course, neither of these chips come cheap. The Core i9-7980XE lives up to its Extreme Edition lineage with an eye-popping $1999 sticker price, and the Core i9-7960X isn't far behind at $1699. These price tags put the highest-core-count Skylake-X CPUs in a somewhat uncomfortable spot for a couple reasons. For one, that kind of money for a CPU is well within workstation-class territory, but neither of these CPUs do anything to address Threadrippers' higher CPU PCIe lane complement or ECC RAM support. Recall that Threadripper CPUs and the X399 platform have both ECC RAM support and 60 PCIe 3.0 lanes directly connected to the CPU. Those resources are available from every Threadripper, too.
In an apparent response to AMD's aggressive marketing of its platform advantages, Intel has begun aggregating the number of PCIe lanes available from both the chipset and CPU in its marketing materials. That aggregation is a bit disingenuous, though, because it doesn't account for the fact that PCIe lanes from the X299 chipset have to traverse the DMI 3.0 link and its roughly 32 Gbps of bandwidth before reaching the host CPU. Some X399 peripheral controllers do need to travel over a similar link on Threadripper systems, but the jockeying for bandwidth from chipset to CPU should be a lot less rowdy there. Even if Threadripper CPUs don't outperform the Skylake-X competition, the robustness of the X399 platform for workstation-class uses remains a point in its favor.
Although we've already discussed the Skylake Server architecture in detail in our review of the Core i9-7900X, the implementation of that architecture in the Core i9-7960X and i9-7980XE is worth exploring a bit more.
It's no secret that Intel has long repurposed server hardware for its high-end desktop processors. The company has made multiple Xeon dies with varying core counts to fit the needs of the businesses it serves, but until now, the company has never had to repurpose its higher-core-count Xeons for duty on the desktop.
We weren't briefed on the various Xeon Scalable Processor dies for the Skylake Server rollout this time around, but this Tom's Hardware report leads us to conclude that the Core i9-7960X and Core i9-7980XE are bringing Intel's high-core-count (or HCC) Skylake Server die down from the data center. Other Skylake-X Core i7s and Core i9s use the 10-core low-core-count, or LCC, die as their foundation. This may be the first time that Intel has ever had to bring its HCC Xeon die to its high-end desktop platform. Competition is a wonderful thing.
Like all Core i9 CPUs, these high-core-count chips boast two AVX-512 execution units: a dedicated AVX-512 unit per core on port five of the unified scheduler, and another created through the fusion of the two AVX-256 units on ports zero and one of the unified scheduler. Recall that the Core i7-7800X and Core i7-7820X are only equipped with one AVX-512 unit per core: the one created through fusion of the dual AVX-256 units. The dedicated AVX-512 unit on port five is disabled on those chips for market-segmentation reasons. Some users have reported that both AVX-512 execution paths are available even on Core i7 products, but we're reporting the official line until we've had time to do some directed testing.
With the release of 12-, 14-, 16-, and 18-core CPUs, the Core i9 lineup is complete. You can see that the highest-end CPUs get a 25W TDP bump over the Core i9-7900X and friends, to 165W. Even with the more generous TDP, some have expressed concern about the clock speeds these higher-core-count Core i9s can hit under load. Happily, my experience with this duo suggests Intel's 2.8 GHz base clock for the i9-7960X and 2.6 GHz base clock for the i9-7980XE are extremely pessimistic for enthusiast desktops with adequate cooling. Intel rates the Core i9-7980XE for 3.4 GHz non-AVX Turbo operation with all cores active, and I can confirm that the chip can hold that speed under a 280-mm liquid cooler. The i9-7960X is rated for an all-core Turbo speed of 3.6 GHz. Here's the full Turbo Boost 2.0 table for each Skylake-X CPU, straight from the horse's mouth:
Typical AVX workloads (albeit not AVX-512) caused the i9-7980XE to fall to just 3.2 GHz per core, but as our performance results will show, that drop hardly matters in the big picture. Regardless, I wouldn't worry about seeing clock speeds under 3 GHz outside of intensive AVX-512 workloads. Given the paucity of programs using those code paths, the average enthusiast shouldn't have any clock-speed worries at stock speeds.
Like most other Skylake-X CPUs, the i9-7960X and i9-7980XE offer an improved Turbo Boost Max 3.0 implementation compared to Broadwell-E. On these high-core-count CPUs, one should see up to 4.4 GHz speeds on two favored cores.
Intel's rebalancing of the cache hierarchy on Skylake Server chips means the i9-7960X and i9-7980XE have massive private L2 caches at their disposal. Each core gets 1MB of L2 to work with, for a total of 16MB on the i9-7960X and 18MB on the i9-7980XE. Recall also that the bandwidth between the L1 and L2 caches on these chips has been increased to 128 bytes per cycle for reads and 64 bytes per cycle for writes. At the same time, the L3 cache per core now serves as a victim cache for the L2 above it, and L3 per core has been cut to 1.375 MB. Contrast that with the 2.5 MB of shared L3 per core on Broadwell Xeons. The new L3 allocation leads to 22MB of L3 across all cores on the i9-7960X and 24.75 MB on the i9-7980XE.
Now that we've revisited the essentials of Skylake-X, it's time to get to testing.